Re: null domains after xl destroy

From: "Roger Pau Monné" <roger.pau@citrix.com>
To: Juergen Gross <jgross@suse.com>
Cc: Steven Haigh <netwiz@crc.id.au>,
	Andrew Cooper <andrew.cooper3@citrix.com>,
	Jennifer Herbert <Jennifer.Herbert@citrix.com>,
	Dietmar Hahn <dietmar.hahn@ts.fujitsu.com>,
	xen-devel@lists.xen.org, glenn@rimuhosting.com
Subject: Re: null domains after xl destroy
Date: Tue, 9 May 2017 10:24:14 +0100	[thread overview]
Message-ID: <20170509092414.uzi6dieevnfi27o3@dhcp-3-128.uk.xensource.com> (raw)
In-Reply-To: <f9ace036-ee19-c93e-7d15-8f5a2a2177ca@suse.com>

On Mon, May 08, 2017 at 11:10:24AM +0200, Juergen Gross wrote:
> On 04/05/17 00:17, Glenn Enright wrote:
> > On 04/05/17 04:58, Steven Haigh wrote:
> >> On 04/05/17 01:53, Juergen Gross wrote:
> >>> On 03/05/17 12:45, Steven Haigh wrote:
> >>>> Just wanted to give this a little nudge now people seem to be back on
> >>>> deck...
> >>>
> >>> Glenn, could you please give the attached patch a try?
> >>>
> >>> It should be applied on top of the other correction, the old debug
> >>> patch should not be applied.
> >>>
> >>> I have added some debug output to make sure we see what is happening.
> >>
> >> This patch is included in kernel-xen-4.9.26-1
> >>
> >> It should be in the repos now.
> >>
> > 
> > Still seeing the same issue. Without the extra debug patch all I see in
> > the logs after destroy is this...
> > 
> > xen-blkback: xen_blkif_disconnect: busy
> > xen-blkback: xen_blkif_free: delayed = 0
> 
> Hmm, to me it seems as if some grant isn't being unmapped.
> 
> Looking at gnttab_unmap_refs_async() I wonder how this is supposed to
> work:
> 
> I don't see how a grant would ever be unmapped in case of
> page_count(item->pages[pc]) > 1 in __gnttab_unmap_refs_async(). All it
> does is deferring the call to the unmap operation again and again. Or
> am I missing something here?

No, I don't think you are missing anything, but I cannot see how this can be
solved in a better way, unmapping a page that's still referenced is certainly
not the best option, or else we risk triggering a page-fault elsewhere.

IMHO, gnttab_unmap_refs_async should have a timeout, and return an error at
some point. Also, I'm wondering whether there's a way to keep track of who has
references on a specific page, but so far I haven't been able to figure out how
to get this information from Linux.

Also, I've noticed that __gnttab_unmap_refs_async uses page_count, shouldn't it
use page_ref_count instead?

Roger.

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